Steel Drain Pipe: Corrosion and Strength of threaded pipe 1

[Mechomatic]

Hello all-

Jumping to the question: It has been suggested that a threaded connection in an inaccessible location between a steel pipe (sch80) and coupling be backwelded to ensure a good seal for years to come. Corrosion and damage due to mishandling during product transport are the main concerns, as the connection will be nearly unserviceable (I know, bad design choice right there). Is backwelding a good idea? Should we just either socket weld it instead of thread the pipe, or would the thread be sufficient to seal and not have to weld it at all?

Thanks for the input!

For those who want the full story:

I'm a fresh-off-the-books ME looking for advice on a redesign I'm doing for my company. I'm changing the design of a bath drain for a hot water bath (actually uses Jeffcool P155 [~50/50 water/glycol with corrosion inhibitors] in service, but the system is tested using tap water) so the drain does not leave standing water in the bath after being drained and put into temporary storage (can be as short as shipping time, may be as long as a few years). The new design will be located where it cannot be reached for service or inspection without getting a crane or group of forklifts to lift the equipment, so it needs to be robust against corrosion and stresses caused from temperature changes and stresses that may be placed on it during transport.

Operating conditions are 1-3 psig with temperature anywhere between ambient to 190F.

Currently, my design is to use a socket weld elbow welded to the drain hole in the bath and sch80 pipe welded into the elbow and threaded to a full coupling to connect to a threaded ball valve via short pipe nipple. All diameters 2".

The main concern is corrosion, as the system may sit unused for some time (before being put into service for lifecycle on the scale of decades), and damage that may occur during transport.

It has been suggested that the threaded connection under that skid between the pipe and coupling be backwelded to ensure a good seal. Is backwelding a good idea? Should we just either socket weld it instead of thread the pipe, or would the thread be sufficient to seal and not have to weld it at all?

 

[metengr]

I would use socket welded or even butt welded fittings versus attempting to back weld pipe threads for sealing purposes.

 

[racookpe1978]

Both back-welding a threaded joint and socket-welding will create crud traps and places for water to collect in a horizontal-jointed pipe. (Much less likely in a vertical pipe with socket-weld joints though.)

Back-welding threaded joints tends to create stress risers, and is not recommended unless you fully weld over the entire visible thread - though not prohibited by most companies. It does mean that , when a leak happens, it is harder to fix since it is not a threaded joint, but is more work than a socket-welded fitting since the whole pipe section has to be pulled.

 

[MechPipe13]

It is probably a good idea to use a blind flange on this case if you do want a seal without the hassles of pitting corrosion on the threads. Over time the valve might develop blockage that might prevent it from opening or closing properly. At least with a two inch blind, you can unbolt and drain. Might even save you money from that valve.